 
// Set desired tab- defaults to four space softtab
var tab = "    ";
       
function checkTab(evt) {
    var t = evt.target;
    var ss = t.selectionStart;
    var se = t.selectionEnd;
 
    // Tab key - insert tab expansion
    if (evt.keyCode == 9) {
        evt.preventDefault();
               
        // Special case of multi line selection
        if (ss != se && t.value.slice(ss,se).indexOf("n") != -1) {
            // In case selection was not of entire lines (e.g. selection begins in the middle of a line)
            // we ought to tab at the beginning as well as at the start of every following line.
            var pre = t.value.slice(0,ss);
            var sel = t.value.slice(ss,se).replace(/n/g,"n"+tab);
            var post = t.value.slice(se,t.value.length);
            t.value = pre.concat(tab).concat(sel).concat(post);
                   
            t.selectionStart = ss + tab.length;
            t.selectionEnd = se + tab.length;
        }
               
        // "Normal" case (no selection or selection on one line only)
        else {
            t.value = t.value.slice(0,ss).concat(tab).concat(t.value.slice(ss,t.value.length));
            if (ss == se) {
                t.selectionStart = t.selectionEnd = ss + tab.length;
            }
            else {
                t.selectionStart = ss + tab.length;
                t.selectionEnd = se + tab.length;
            }
        }
    }
           
    // Backspace key - delete preceding tab expansion, if exists
   else if (evt.keyCode==8 && t.value.slice(ss - 4,ss) == tab) {
        evt.preventDefault();
               
        t.value = t.value.slice(0,ss - 4).concat(t.value.slice(ss,t.value.length));
        t.selectionStart = t.selectionEnd = ss - tab.length;
    }
           
    // Delete key - delete following tab expansion, if exists
    else if (evt.keyCode==46 && t.value.slice(se,se + 4) == tab) {
        evt.preventDefault();
             
        t.value = t.value.slice(0,ss).concat(t.value.slice(ss + 4,t.value.length));
        t.selectionStart = t.selectionEnd = ss;
    }
    // Left/right arrow keys - move across the tab in one go
    else if (evt.keyCode == 37 && t.value.slice(ss - 4,ss) == tab) {
        evt.preventDefault();
        t.selectionStart = t.selectionEnd = ss - 4;
    }
    else if (evt.keyCode == 39 && t.value.slice(ss,ss + 4) == tab) {
        evt.preventDefault();
        t.selectionStart = t.selectionEnd = ss + 4;
    }
}
 

